Polyether polymers such as polyethylene oxide and polypropylene oxide exhibit a high ionic conductivity when an electrolyte salt compound is incorporated therein. Therefore, polyether polymers are known as a material suitable for a solid polymer electrolyte. Especially a solid polymer electrolyte film made of a polyether polymer is used for a lithium ion battery.
For example, a proposal has been made in Japanese Unexamined Patent Publication No. S63-136407 wherein an ethylene oxide-propylene oxide copolymer having mixed therewith an alkali metal salt is used as an ionic conductive solid electrolyte. More specifically it is described in the patent publication that a film exhibiting a high ionic conductivity is made by a process wherein a solution of an ethylene oxide-propylene oxide copolymer and LiCF3SO3 in acetonitrile is cast into a film and then acetonitrile is removed from the as-cast film in a stream of inert gas and under a reduced pressure.
As polymer batteries have spread wide in recent years, it is required to produce solid polymer electrolyte films with enhanced industrial productivity. However, polyether polymers are generally subject to molecular weight degradation due to polymer molecule scission. That is, the molecular weight of the polymer is easily reduced, for example, while the polymer is stored as a material for shaping, or when the polymer is melt-extruded into a film or the polymer is molded. Therefore a film having a high mechanical strength is difficult to obtain. To solve this problem, the present inventors made an attempt for incorporating an antioxidant in a polyether polymer when the polymer is melt-extruded into a film, but it was difficult or even impossible to suppress the reduction of molecular weight to a desired extent.
A proposal has been made, for example, in Japanese Unexamined Patent Publication No 2000-123632 wherein ethylene oxide and propylene oxide are copolymerized together with ally glycidyl ether, and a resulting copolymer is crosslinked with a peroxide to prepare a crosslinked copolymer capable of giving a film having high ionic conductivity, good flexibility and high mechanical strength. However, the present inventors have confirmed that the polyether copolymer is undesirably crosslinked to be thereby partially gelled while the copolymer is stored before the melt-extrusion for film-formation, and therefore the copolymer is difficult to shape, and, even if the copolymer can be shaped, a resulting film has poor mechanical properties.